Cyanate ester (CE) resin was modified in a multi-component way using the technology of conjugated tri-component interpenetrating polymer networks (CTC-IPN). Experimental results showed that the three components in the modified composite existed undependably and they interweaved irregularly in the structure of IPN to form a unique supermolecule structure. The analysis by differential scanning calorimetry (DSC) indicated that the Tg of the modified CE was increased by 22°C, compared with the Tg prior to modification. When the amount of an initiator was 0.3% or 0.4% and concurrently the ratio of PST/PMMA/CE was 15/15/85, the flexural strength and the impact strength reached the maximum values. Compared with those of pure CE, the flexural strength was increased by 22.65% and the impact strength by 94.99%, respectively.
Interpenetrating polymer networks (IPN) is a kind of interweaved net polymer formed by the interpenetration of two or more than two kinds of cross linked polymers [1–5]. This kind of polymer is different from the usual mechanical blending polymer and the graft copolymer. It is the cross linked interaction of different components, interfingering through the polymer network. In general, IPN is not the cross link at the molecular level, but the interpenetration at the supramolecular level. IPN can improve the compatibility of various molecular chains, enhance the network density, and micronize the phase microstructure to improve the binding force between phase and phase. Thus, the structural changes and phase structure that a single polymer does not have are achieved. The mechanical properties and other properties are eventually improved [5,6].
In 1960, Millar first proposed the concept of IPN [3,7,8]. Thirty years later, Jia Demin, on the basis of this concept, explained the connotation of the conjugated three-component interpenetrating polymer networks (CTC-IPN) [9–11]. He thought that CTC-IPN is a unique ternary interpenetrating polymer network system, of which one polymer is acted as a public network and two other linear polymers, not interpenetrating but contacting closely, penetrate each other and entangle in the public network. This way, three polymers are closely combined to give CTC-IPN composite a special structure, morphology and properties [12,13]. This special interface structure of CTC-IPN is called the interface conjugated interpenetration [7,11,14].
In this work, cyanate ester resin was modified using a CTC-IPN interface conjugated interpenetrating method. Bisphenol A type isocyanate resin (BADCy) was used as a public network to interpenetrate and entangle each other with methyl methacrylate (PMMA) and polystyrene (PST) to finally prepare an interface conjugated three-component interpenetrating polymer composite. The composite prepared was characterized by infrared spectrometer (IRS) and differential scanning calorimetry (DSC). The mechanical properties of the composite were carefully investigated.2Experimental2.1Experimental raw materials and pretreatment
Bisphenol A cyanate ester resin (industrial product, 98% in purity, white crystal, melting point 79°C) was made in Ji’nan, China. Prior to experiment, it was dried for subsequent preparation at the temperature between 50 and 60°C. Methyl methacrylate (MMA), analytical reagent, was made in Tianjin Chemical Reagent Factory No.6, China. It was washed by alkali (0.1mol/L NaOH) in atmospheric distillation. Styrene (ST), chemical pure, was from Tianjin Dagang Chemical Plant, China. It was washed by alkali (0.1mol/L NaOH) at atmospheric distillation. Azobisisobutyronitrile (AIBN), chemical pure, was purchased from Shanghai Four Hervey Chemical Co., Ltd. Epoxy resin, bisphenol A type (two glycerine ether DGEBA), epoxy value 0.52, industrial product, was made in Wuxi Di’aisheng Epoxy Co., Ltd.2.2Synthesis of interfacial conjugated interpenetrating polymer network
In the presence of trace evocating agent AIBN, MMA and ST were pre-polymerized, respectively, at the temperature between 60 and 70°C for 2h. When the reagent turned slightly thickened, a quantitative BADCy melt was added with the temperature around 60°C. A homogenizer was employed to stir the mixture evenly. The mixture was transferred into a preheated mold and placed in a thermostat of the constant temperature of 70°C for 1h. The box was vacuumed to remove air bubbles. According to the curing process of 120°C/2h–150°C/1h–180°C/1h–200°C/4h, the mixture was polymerized and solidified, cooling naturally to room temperature.2.3Testing instruments and characterization of performances
The impact strength and the bending strength were measured, respectively, in German DL-1000B and XCL-40 material testing machines in accordance with the specifications of GB3357-82 and GB3356-82. The deviation was controlled within less than 10%. The sample size was 15mm×10mm×4mm. The coating method and the KBr compression method were adopted for infrared spectrum analysis, using a WQF-310 infrared spectrometer made in Beijing Second Optical Instrument Factory. The glass transition temperature was characterized by differential scanning calorimetry in accordance with the national standard GB1634-79. It was measured by a Perkin-Elmer DSC thermal analyzer in nitrogen atmosphere. The starting temperature was 50°C and the heating rate was increased by 10°C/min.3Results and discussion3.1Comparison of mechanical properties of pure CE and interfacial conjugated interpenetrating polymer network
Table 1 indicated the mechanical properties of the modified system under different formulations. As can be seen, except A formula, the impact strength of the PST/PMMA/CE in different mass ratios was all superior to that of pure cyanate ester resin. This is mainly because that cyanate ester resin, methyl methacrylate, and polystyrene interpenetrated each other, entangling to form an interface conjugated interpenetrating network. This network played a role of “forced inclusion” and “synergetic effect” [7,12].
Comparison of mechanical properties between interfacial conjugated inter-penetrating polymers and pure CE under different formulations.
|No.||Formulation||CE/MMA/ST (mass ratio)||Impact strength/KJm−2||Flexural strength/MPa|
Cyanate ester is a thermosetting resin. Its structural formula is NCORCN, where R stands for
CE was in situ polymerized (curing) into polycyanurate (PCN) at high temperature under the action of catalyst. PCN has many advantages of good toughness, higher glass transition temperature (Tg), lower dielectric constant and loss factor, lower water absorption rate, and inherent flame retardancy. At high temperature (220–270°C), its molten body is chemically stable. At the high temperature of 250°C, it still has excellent bonding on metal conductors [3,7,11].
After physical modification of CE, the so-called semi interpenetrating network (IPN) was formed. This network can also make CE consolidated into PCN. The disadvantage of physical modification is that the achieved PCN is heterogeneous, the regional structure of each phase is difficult to control, and the processing performance is poor. The curing degree of CE depends on the final morphology of the polymer blends.
The main factors that affect the performances of IPN are the interpenetrating degree, component ratio, and cross link density. Due to the enhancement of the interpenetration between the three components and the decrease of the density of the single component such as CE, the impact strength and bending strength of the network polymer are obviously improved, both of which are consistent with the experimental design.3.2Influence of MMA/ST contents on the mechanical properties of IPN
Fig. 1 demonstrated the changes of the mechanical properties of IPN with different contents of ST/MMA. Obviously, in the system of different ratios of ST/MMA/CE, both the flexural strength and the impact strength of IPN were increased first and then decreased sharply with the gradually increasing mass ratios of MMA and ST as shown in Fig. 1(a) and (b). The maximum values of both appeared at the content of CE 85%. They reached the optimum state in the mass ratios of CE/MMA/ST 85/15/15. Compared with the pure CE, the bending strength increased by 22.65%, whereas the impact strength was improved by 94.99%. This is mainly because, with the increase of the contents of MMA and ST, the system transformed gradually from the single CE phase into the miscible system of PMMA and PST assisted phase. The interpenetrating extent of CE with PMMA and PST was gradually increased, improving the rigid dense structure of the pure CE macromolecular network to form a system of interpenetrating interface conjugated supramolecular structure. As a result, the purpose of toughening modification was realized.
However, when the contents of MMA and ST were continuously increased, the compatibility between phase and phase was gradually declined, the self aggregation tendency of each component was enhanced, and the supramolecular structure was gradually weaken. Meanwhile, the main featured structure of CE suffered a greater degree of impact. As a result, both the impact strength and the bending strength were decreased significantly.3.3Effect of initiator dosage on the mechanical properties of IPN system
Fig. 2 indicated the effect of AIBN contents on the mechanical properties of the composite. When the amount of initiator (against the mass of cyanate ester resin) was 0.3%, the flexural strength of the composite reached the maximum value as shown in Fig. 2(a). When it was lower or higher than 0.3%, the flexural strength was declined. When the amount of AIBN was 0.4%, the impact strength of the IPN system reached the maximum value seen Fig. 2(b). In the case of the amount of AIBN being below or above that value, the impact strength was reduced. The variations were due to the reason that when the concentration of the initiator was at a certain proper value, the polymer chain formed by the pre-polymerization of MMA and ST was wrapped around the network formed by CE. In this situation, PST and PMMA had larger activity space. The CE macromolecular network structure characteristics were slightly influenced. At this moment, the synergistic effect was dominated. Together with the cyanate resin net, CE shared the impact from the external force, enhancing the strength and toughness of the cyanate ester resin.
When the amount of the initiator was too large, the fast reaction between PMMA and PST took place. Prior to interpenetrating and entanglement, the reaction was completed. Thus, the modification was unable to be realized. But when the amount of the imitator was too small, the reaction speed was too slow. In this case, a linear polymer chain segment of ideal polymerization degree was unable to form within the effective time. The interpenetrating and winding process was impossible to be completed. It was difficult to form the polymer of the interface conjugated supramolecular structure. So, IPN cannot play its normal role, reducing greatly the effect over the toughness of CE [9,13].3.4Analysis by infrared spectrum
The wavenumber of the characteristic absorption of each group in the cyanate resin was shown in Table 2. From the infrared spectroscopy in Fig. 3(a), it can be seen that, before and after modification of the cyanate resin, there were two stronger cyanogen doublets at 2235cm−1 and 2270cm−1, respectively, indicating that there existed cyanogens in the semi cured product. A weak absorption peak appeared at between 3400cm−1 and 3600cm−1, showing that there was a small amount of hydroxyl group that was not fully reacted. At 1750cm−1, a carbonyl absorption peak occurred, signifying that cyanate was reacted partially to generate urethane or imino carbonic acid ester. At 1369cm−1, the absorption peak was narrowed, demonstrating that there was polycyanate. At 1565cm−1, a strong absorption peak appeared, which proved the existence of triazine ring. It can be inferred that the system was partially polymerized and the polymerization reaction of forming triazine ring took place.
Comparing Fig. 3(a) with (b), it can be seen clearly that the content of cyanate ester in the IPN system was decreased significantly at 2270cm−1, whereas the content of triazine ring was increased apparently at 1560cm−1. This variation showed that at low temperature the self polymerization of cyanate ester occurred mainly to produce the structure of triazine ring. In this case, the characteristic functional groups were still clearly visible in Fig. 3(a) and (b). In terms of pure cyanate resin, there was very slight difference in the two figures. The only small difference lied in the appearance of functional groups of other components, which indicated that, in the curing process, chemical bonds were formed independently in the three components. They interpenetrated and entangled CE through linear polymers, constituting unique CTC-IPN supramolecular structure .3.5Analysis of differential scanning calorimetry
Fig. 4 showed the Tg spectra of the glass transition temperature, from which it can be seen that the Tg of pure CE was at 214°C. After modification, the Tg of the PST/PMMMA/CE composite was 236°C, increasing by 22°C. In addition, the graph after modification was similar to that of pure CE. Only one glass temperature appeared, which showed that a unique interface conjugated IPN system was fully formed through the interpenetrating and entanglement of the three components as well as through the interface conjugation. No phase separation took place.
The characteristics of a single phase always maintained such that the performances of CE modified system were optimized obviously. Eventually, the heat resistance of the composite was greatly improved.3.6Corrosion resistance of the composite
The PST/PMMMA/CE composite can be used as parts of aviation equipment [1,3,6,7]. In the aerospace environment, the components often suffer from acid, alkali, ultraviolet and other corrosion. Therefore, there are high requirements for the curing material against the chemical resistance and highlight corrosion. To improve the resistance against corrosion, a sample was soaked in 10% HCl solution, 6mol/L H2SO4 solution, and 20% NaOH solution for 18 days. Then, the changes were observed and the amount of losses was measured.
The experimental results showed that 10% HCl solution did not have a corrosive effect on the cured product, while 6mol/L H2SO4 solution and 20% NaOH solution had a certain degree of corrosion. The test results were shown in Table 3.
From the experimental results, it is known that, compared with pure CE, the corrosion degree of the solidified product was gradually reduced with the increase of the contents of ST and MMA. As a whole, the mass loss of the IPN system was less than 1%, which can meet the requirements of the aerospace environment. This result is directly related to the introduction of PMMA/PST. When PMMA/PST was introduced, two macromolecular chains crossed and entangled to cover and reinforce the local non-aggregation point of the CE network, leading eventually to more complete polymerization degree. As a result, the corrosion resistance of the interpenetrating polymer network composite was enhanced.4Conclusions
After CE was modified by the CTC-IPN interfacial conjugated interpenetrating method, both the mechanical properties and the glass transition temperature of the achieved PST/PMMA/CE composite system were improved significantly. The interface conjugated interpenetrating polymer network that was synthesized in accordance with the process of 120°C/2h–150°C/1h–180°C/1h–200°C/4h exhibited the best mechanical property at the ratio of ST/MMA/CE=15/15/85. Compared with the pure CE, the impact strength was increased by 69.99%, while the bending strength by 12.92%.
IRS analysis and DSC results showed that the three components coexisted independently. Through interpenetrating and entanglement, they were together with CE to form the interface conjugated supramolecular network system. This way, Tg was increased by 22°C and the IPN system of unique structure was constituted. At the same time, the initiator AIBN exerted a certain influence on the mechanical properties of the system.
The authors declare no conflicts of interest.
The authors would like to express their gratitude for the financial support by Shaanxi Province Engineering Laboratory of High Performance Concrete with the grant No. G2015-03 and No. G2015-05 as well as by Weinan Basic Research Innovation Talent Project with the grant No. 2015KYJ-3-3. The authors would also like to thank Prof. Li Kezhi of Northwestern Polytechnical University for his experimental support.